Am Donnerstag 15 April 2010 00:49:26 schrieb Patrick LeBoutillier:
Hi,
Sorry I can't be of much help here, but I have a question though:
data StateCmd a where Put :: a -> StateCmd a Undo :: StateCmd a Redo :: StateCmd a deriving (Show, Functor)
I'm not familiar with the above syntax (data ... where).
It's GADT syntax, read about it in the user's guide.
What does it do exactly?
That is exactly equivalent to data StateCmd a = Put a | Undo | Redo deriving (...) The coolness of GADTs is that you can have different constructors in your datatype for different type parameters: data Expr p where Plain :: a -> Expr a Add :: Num a => Expr a -> Expr a -> Expr a If :: Expr Bool -> Expr a -> Expr a -> Expr a And :: Expr Bool -> Expr Bool -> Expr Bool ... And you can pattern match on these constructors for different types in one function: eval :: Expr a -> a eval (Plain x) = x eval (Add e1 e2) = eval e1 + eval e2 eval (If eb t f) = if eval eb then eval t else eval f eval (And e1 e2) = eval e1 && eval e2 ...
Patrick